1. Field of the Invention
This invention relates generally to electrical circuits for piezoelectric elements and, more particularly, to efficient drive circuits for a capacitive load such as a piezoelectric element.
2. Description of the Related Art
In applications where a piezoelectric element or similar capacitive-type load is coupled to a drive circuit, it is desirable to produce a high output to the load without wasting a great amount of energy. Piezoelectric elements are commonly used to generate an audible alarm by driving them with an AC voltage, which causes the piezoelectric elements to vibrate at their natural oscillation frequency and in time with the voltage, producing a sound. This is especially useful where it is important that the circuit elements not take up much space, such as in a heart pacemaker. A piezoelectric element is smaller and requires less energy to drive than an electromagnetic device, and can be supplied with energy from very compact batteries. The piezoelectric element also can be used in an implanted drug pump, the vibrations of the element being used to initiate the flow of a drug into the body.
The efficiency of the drive circuit can be increased, and the amount of energy dissipated as heat or otherwise lost can be decreased, by using a resonant drive circuit where the piezoelectric element, or capacitive load, is connected in series with an inductive element. The two form what is known as an L-C circuit, in which an inductor (L) and capacitor (C) comprise reactive elements whose impedances change with the frequency of an applied voltage. An L-C circuit has a resonant frequency at which the reactive impedances of the inductor and capacitor exactly cancel each other, and at which maximum energy transfer occurs. As a consequence, the L-C circuit comprises a resonant energy transfer system that can produce an instantaneous output voltage to the capacitor many times larger than the supply voltage with minimal energy lost as heat. The frequency at which resonance occurs will depend on the inductance and capacitance values.
In an L-C resonant drive circuit, if the piezoelectric element is to be used to produce an audible signal, the element must vibrate in the audible frequency range, preferably at approximately 1 kHz. That is, the supply voltage must be applied at this frequency. Typical piezoelectric elements that vibrate in this frequency range and that are suitable for implantation have capacitance values for which corresponding inductors are unfortunately too large to be implanted. Moreover, an inductor small enough to be implanted, when combined with a typical piezoelectric element, would correspond to a resonant frequency on the order of 100 kHz, well above the frequency range of human hearing.
From the discussion above, it should be apparent that there is a need for a resonant drive circuit for use with a piezoelectric element vibrating at an audible frequency and providing high energy efficiency while being of implantable size. The present invention satisfies this need.